If you are only infected, then you have to sleep in a bed to become a vampire officially. If you are a vampire then yes, you can feed on sleeping people to satisfy the hunger, or you can let the hunger increase and become stonger at the expense to being weaker to the sun and fire if I'm correct. If you don't feed though, many shop keepers won't deal with you, so to buy and sell you need to feed.

Go to Northwatch Keep kill all the thalmor there(note they do not respawn)there is a prisoner sleeping you can keep going back end of each day have 3 or 5 feeds and leave. Note if she wakes up don't talk and leave when you go back she is asleep again. You can then level up to master doing this (quick way is press T to pass the days) Note after a feed you can go into towns etc. This is in vanilla mode.

Should be two standing and a woman sleeping on the floor in one of the cells. the cells are close you open them they just stand there the other just sleeps. Have you done the old girl (jewelly) in Whiterun market request, if you haven't maybe go back a few saves and redo some of the game won't spoil to much, but if you have done it, she should be there(I think it is a gitch in the game) and they miss respawn there, any other fort you go into there are alway someone to fight.